Nitzsche and Wick, U.S. Pat. No. 3,065,194 disclose a family of silicone rubber compositions comprising an essentially anhydrous mixture of (1) a linear organosiloxane polymer having hydroxyl terminal groups, (2) a polyfunctional organosilicon cross-linking agent, and (3) a metal salt, chelate, organometallic compound, acid or base, which serves as a cross-linking catalyst. Such compositions vulcanize or cure to rubbery solids when exposed to moisture and this makes them uniquely useful, in that they can be maintained over a long period of time in a single container, e.g., sealed in caulking tubes, ready for use, but then the user can apply the material and bring it into contact with water or water vapor to cure it. Such compositions are useful in sealants, electrical insulation, coatings, dental cement, caulking compounds, expansion joints, gaskets, shock absorbers, adhesives, and in many other forms. In general, the Nitzsche et al patent states that the compositions can contain 100 parts of siloxane polymer, 0.1 to 50 parts of cross-linking agent and 0.01 to 10 parts of cross-linking catalyst. The ratio of the cross-linking agent to the cross-linking catalyst is not specified, but in the working examples, which involve silanes as the cross-linking agents and metal salts as catalysts, the ratios vary from less than unity to greater than unity.
Additional background teachings relating to one-package room temperature vulcanizing silicone compositions are to be found in Brown et al, U.S. Pat. No. 3,161,614, who show a pre-reacted silanol end-stopped diorganopolysiloxane and cross-linker, in combination with a cross-linking catalyst; Cooper, U.S. Pat. No. 3,383,355, who deals with the preparation of an alkoxy-terminated linear siloxane polymer using a neutral, finely divided solid catalyst, e.g., fuller's earth; Matherly, U.S. Pat. No. 3,499,859, who uses a hydrocarbonoxy end-blocked diorganopolysiloxane and a metal-containing curing catalyst along with boron nitride; and Cooper et al., U.S. Pat. No. 3,542,901, who use a mixture of a linear siloxane having di- or tri-functional end-blocking units with a linear siloxane having chemically non-functional inert end-blocking units on one end and di- or tri-functional end-blocking units on the other, and include a catalyst and a cross-linker. Also of interest are Brown et al., U.S. Pat. No. 3,122,522, who combine organopolysiloxane intermediates containing condensable cellosolvoxyl groups with a catalyst; Brown et al, U.S. Pat. No. 3,170,894, who combine organopolysiloxane intermediates containing condensable polyhydrocarbonoxy type radicals with a catalyst; and Weyenberg, U.S. Pat. No. 3,175,993, who combines organopolysiloxane intermediates end-blocked with alkoxylated silcarbane groups with a catalyst.
Smith and Hamilton, U.S. Pat. No. 3,689,454 and U.S. Pat. No. 3,779,986, Weyenberg, U.S. Pat. No. 3,294,739 and 3,334,067, and Clark et al., U.S. Pat. No. 3,719,635 provide one-package compositions with the same advantages as those of Nitzsche et al, but, because the former use a titanium ester catalyst instead of the metal salts of Nitzsche et al., it is easier to control gelation and the undesired build-up in viscosity during mixing and the compositions can be stored for longer periods of time, before use. Although these citations disclose use of cross-linkers and titanium ester catalysts over a broad range of compositions, the ratios between these components are not particularly specified, and in all of the working examples, the ratio of cross-linker to catalyst is always in excess of 2. Moreover, both Smith and Hamilton and Weyenberg state a preference for the presence of an amount of silane in excess of the silicon bonded hydroxyl -- to protect the system from gelation -- and the amount of catalyst in their work is always selected to be less than one-half of the amount of cross-linker used.
Although a wide choice of components is thus seen to be useful for the preparation of one-package RTV compositions, there has not been too much appreciation by those skilled in the art of the factors involved in securing specified properties in the cured silicone rubber. It is known, for example, that some variations in properties can be induced in the cured composition by varying the molecular weight of the silanol chain-stopped polydiorganopolysiloxane. For example, as the molecular weight is increased, elongation of the rubber increases. On the other hand, if a lower viscosity material is used, the cure is tighter so that the cured material has a lower elongation and increased hardness. The adhesion of the cured composition to a variety of conventional substrates is also a factor which can be predictably influenced and this generally has involved incorporating a monofunctional silane chain-terminating agent in combination with the cross-linker, or in extreme cases, by adding an adhesion promoter. In any event, both modulus control and adhesion control are not easily achieved and still leave a lot to be desired in the present state of the art.
It has now been discovered that if the conventional ingredients are used, but at properly selected and novel ratios, self-bonding, low modulus, one-package room temperature vulcanizing silicone rubber compositions can be obtained with a very advantageous tensile and elongation relationship, i.e., low tensile and exceptionally high elongation. Moreover, compositions are provided which give self-bonding adhesion properties to many ordinarily troublesome substrates, with less need to use adhesion promoters or even permitting them to be eliminated from the composition.
These results are unexpected, because the present compositions use the catalyst and the cross-linker in a ratio of at least 0.5 to 1 (instead of less than 0.5 to 1, i.e., more than a 2-fold excess of the cross-linker as in the prior art) and, in the most preferred compositions, the ratio of catalyst to cross-linker will always be at least unity. In preferred cases, the silane cross-linker will be used in amounts less than that previously thought necessary to cover all of the silanol groups in the base product and prevent gelation. If the metal ester catalyst is present in an amount greater than one-half the weight of the silane, no such gelation has been seen to occur.